Adds some tests for project structure verifier

build.sbt

@@ -5,5 +5,6 @@ lazy val root = project
     name := "module-layers",
     organization := "com.github.tammo",
     version := "0.1.0-SNAPSHOT",
-    scalaVersion := "2.12.18"
+    scalaVersion := "2.12.18",
+    libraryDependencies += "org.scalatest" %% "scalatest" % "3.2.17" % Test
   )

src/main/scala/com/github/tammo/verification/ProjectStructureVerifier.scala

@@ -7,9 +7,11 @@ object ProjectStructureVerifier {
   def verify(
       module: Module,
       architectureStyle: ArchitectureStyle
-  ): Iterable[Violation] = {
+  ): Set[Violation] = {
     val potentiallyAllowedLayers =
-      architectureStyle.permits.foldLeft[Option[Set[Layer]]](Some(Set.empty)) {
+      architectureStyle.permits.foldLeft[Option[Set[Layer]]](
+        Some(Set.empty)
+      ) {
         case (None, _) => None
         case (result, PermitAll(layer)) =>
           if (module.layer == layer) {
@@ -28,7 +30,7 @@ object ProjectStructureVerifier {
     potentiallyAllowedLayers match {
       case None =>
         // None expresses the absence of forbidden layers
-        Iterable.empty
+        Set.empty
       case Some(allowedLayers) =>
         module.dependencies.flatMap(
           isDependencyViolated(module, _, allowedLayers)

src/test/scala/com/github/tammo/verification/ProjectStructureVerifierTest.scala

@@ -0,0 +1,75 @@
+package com.github.tammo.verification
+
+import com.github.tammo.ModuleLayersPlugin.{Aggregate, NoArchitectureStyle}
+import com.github.tammo.domain.ModuleLayersDomain.*
+import org.scalatest.flatspec.AnyFlatSpecLike
+import org.scalatest.matchers.should.Matchers
+
+class ProjectStructureVerifierTest extends AnyFlatSpecLike with Matchers {
+
+  private val emptyModule = Module("empty", DependencyLayer, Set.empty)
+
+  private val moduleWithDependency =
+    Module("with-dependency", Aggregate, Set(emptyModule))
+
+  private case object DependencyLayer extends Layer
+
+  private def architectureStyleForTest(
+      testPermits: Permit*
+  ): ArchitectureStyle = new ArchitectureStyle {
+    override def styleName: String = "test-style"
+
+    override def permits: Iterable[Permit] = testPermits
+  }
+
+  it should "have no violation if module has no dependencies" in {
+    val violations =
+      ProjectStructureVerifier.verify(emptyModule, NoArchitectureStyle)
+
+    violations shouldBe Set.empty
+  }
+
+  behavior of "explicit permit"
+
+  it should "have no violations if module with dependency has explicit permit" in {
+    val architectureStyle: ArchitectureStyle =
+      architectureStyleForTest(Aggregate --> DependencyLayer)
+
+    val violations =
+      ProjectStructureVerifier.verify(
+        moduleWithDependency,
+        architectureStyle
+      )
+
+    violations shouldBe Set.empty
+  }
+
+  it should "have violation if module with dependency has explicit permit for other layer" in {
+    case object OtherLayer extends Layer
+
+    val architectureStyle: ArchitectureStyle =
+      architectureStyleForTest(Aggregate --> OtherLayer)
+
+    val violations =
+      ProjectStructureVerifier.verify(
+        moduleWithDependency,
+        architectureStyle
+      )
+
+    violations shouldBe Set(
+      Violation(moduleWithDependency, emptyModule, Set(OtherLayer))
+    )
+  }
+
+  behavior of "permit all"
+
+  it should "have no violations if module with dependency has permit all" in {
+    val architectureStyle = architectureStyleForTest(PermitAll(Aggregate))
+
+    val violations =
+      ProjectStructureVerifier.verify(moduleWithDependency, architectureStyle)
+
+    violations shouldBe Set.empty
+  }
+
+}